This talk was given at a yoga retreat held by the Sivananda Yoga Vedanta Centre in Val Morin, Québec (Canada), probably in 1978 after Marcel Vogel's return from his trip to India. This trip is discussed in chapter 3 of Dr Vogel's unfinished (and for a long time unpublished) autobiography. The retreat was led by Swami Vishnu Devananda, whom Marcel Vogel calls swamiji during this talk.